Power conversion devices that step up DC voltage inputted from a DC power supply by a DC/DC converter, convert the resultant voltage to AC voltage by an inverter, and output the AC voltage, are often used for a stand-alone power supply, a UPS (Uninterruptible Power Supply), and the like. In such a power conversion device, the DC/DC converter constantly performs switching operation, and the inverter also constantly performs switching operation.
Also, by using a three-phase inverter, voltage of the DC power supply can be converted to three-phase AC voltage (for example, see Patent Literature 1 (FIG. 7)).
FIG. 16 is an example of a circuit diagram of a power conversion device used in the case of supplying power from a DC power supply to a three-phase AC load. In FIG. 16, a power conversion device 200 generates AC power on the basis of DC power received from a DC power supply 201, and supplies the power to a three-phase AC load 220.
The power conversion device 200 includes: a capacitor 202; for example, three step-up circuits 203; a smoothing circuit 205 for smoothing voltage of a DC bus 204; a three-phase inverter circuit 207; and three pairs of AC reactors 208 to 210 and capacitors 211 to 213. The smoothing circuit 205 is formed by connecting two capacitors 206 in series for the purpose of obtaining the withstand voltage property and connecting six sets of such two capacitors 206 in parallel for the purpose of obtaining the capacitance. The capacitance of the smoothing circuit as a whole is several mF, for example.
The step-up circuit 203 steps up voltage which has been caused to have a high frequency through switching, by an isolation transformer 203t, and then rectifies the stepped-up voltage. The three step-up circuits 203 are connected in parallel to the common DC bus 204. The outputs of the three step-up circuits 203 are smoothed by the smoothing circuit 205 having a large capacitance, to become the voltage of the DC bus 204. This voltage is subjected to switching by the three-phase inverter circuit 207, thereby generating three-phase AC voltage including a high-frequency component. The high-frequency component is removed by the AC reactors 208 to 210 and the capacitors 211 to 213, whereby three-phase AC voltage (or power) that can be provided to the three-phase AC load 220 is obtained. The line-to-line voltage of the three-phase AC load 220 is 400V.
Here, the voltage of the DC bus 204 is required to be equal to or higher than the wave crest value of AC 400V, which is 400×√2, i.e., about 566V, but is set at 600V, considering some margin. In the case where the voltage of the DC bus 204 is 600V, when a switching element in the three-phase inverter circuit 207 is turned off, due to resonance by a floating inductance and the capacitance of the switching element, voltage that greatly exceeds 600V is applied to the switching element. Therefore, in order to reliably prevent insulation breakdown of the switching element, for example, withstand voltage property of 1200V which is twice as high as the voltage of the DC bus is required. In addition, the withstand voltage property of 1200V is also required for the smoothing circuit 205, and in the configuration in FIG. 16, withstand voltage property of 600V is required for each capacitor.